1. Field of the Invention
The present invention is directed to an ethanol production system and method and, in particular, to an ethanol production system and method with increased efficiencies through two stage fusel oil scrubbing.
2. Description of the Prior Art
Ethanol production has increased as demand for ethanol has increased. Plants for ethanol production have been developed to meet the demand. Although ethanol provides an alternative energy source and can be blended with other fuels, its production has challenges for efficiencies related to energy consumption, water usage, emission limits and pollution control.
FIG. 1 shows an exemplary prior art ethanol production system 100. The system 100 includes a fermentation tank 110, a slurry mix processor 112, and a beer well 115. A starch or cellulose-containing feedstock, such as corn, sorghum, barley, wheat, potatoes, sugar cane, agricultural residues etc, is liquefied in the slurry mix processor 112 by mixing the feedstock with water. Other inputs 113 may be added into the slurry mix processor 112, such as enzymes. A slurry mix pump 114 transports the slurry mix from the slurry mix processor 112 to the fermentation tank 110. The fermentation tank 110 converts sugar contained in the slurry to ethanol using a micro-organism such as yeast. Hazardous air pollutants and a beer product are produced in the fermentation tank 110. The beer product or beer is stored in the beer well 115 prior to distillation.
The exemplary prior art ethanol production system 100 further includes a beer column 120, a rectifier column 122, a stripper column 124, and a molecular sieve 126. Beer stored in the beer well 115 is transported by a beer pump 116 to the beer column 120 via a heat exchanger 118 for increasing the temperature of the beer. The beer column 120 is an initial distillation of ethanol in which solid components or stillage are removed from the liquid components of the beer. The liquid components of the beer are then transported to the rectifier column 122 in which ethanol reaching 95% purity is distilled by further removing water. The 95% ethanol is dehydrated in the molecular sieve 126, e.g. by using a desiccant that captures water molecules and allows ethanol molecules to pass through. The dehydrated ethanol, which may reach 99.4% purity, is then stored. The stripper column 124 is used to remove any residual ethanol from the bottoms of the rectifier column 122. This ethanol and water mixture is returned to the rectifier column 122.
The system 100 further includes a CO2 scrubber 130 and a vent gas scrubber 140. The air pollutants produced during fermentation are processed in the CO2 scrubber 130 using fresh water as scrubber solvent. The CO2 scrubber also captures ethanol vapors and mists that are returned to the process through the process condensate storage tank. The cleaned emissions are released into the atmosphere and water slurry is returned to the process to the slurry mix processor 112 via a process condensate storage tank 150 and a pump 155. Vent gas from the beer column 120 is processed in the vent gas scrubber 140, in which the vent gas is scrubbed with fresh water. The cleaned gas is released into the atmosphere and the water slurry is returned to the slurry mix processor 112 via the process condensate storage tank 150 and the pump 155.
As shown in FIG. 1, some ethanol production processes return a portion of the ethanol produced to the beginning stage of the process. As much as 0.5% ethanol has been reported in water entering the fermentation process. By returning the ethanol to the start of the process, the energy that is used to advance the ethanol through the distillation process is forfeited, leading to inefficiencies. Moreover, the fermentation process is limited by the toxicity posed by ethanol to the yeast in the fermentation process. As the concentration of ethanol increases, up to a typical maximum yield of 15%-18%, the yeast will begin to die off. This may have the effect of essentially adding ethanol back into the start of the production process, thereby limiting the production capacity of the facility.
Other issues arise with the control of acetaldehyde. To control the amount of acetaldehyde created and minimize hazardous air pollutants, sodium bisulfate or ammonium bisulfate has been used to increase the solubility of the acetaldehyde in the scrubber water. The addition of these substances has increased control from an initial 10%-30% solubility by up to 50%-70% solubility. The use of bisulfate additives in the CO2 scrubber has been linked to high sulfate concentrations in the dehydrated ethanol. The sulfate concentration of dehydrated ethanol is limited by the customers of the dehydrated ethanol
It can be seen that a need exists to improve the efficiency of ethanol production. Moreover, such a system and process should capture and control hazardous air pollutants emitted from the process, increase production capacity and energy efficiency, eliminate the use of bisulfate scrubber additives and thereby reduce sulfate concentration in the dehydrated ethanol product. The present invention addresses these as well as other problems associated with ethanol production.